Biotic and abiotic isotope fractionation of copper and iron : From the lab to the field scale

Abstract: The distribution of the stable isotopes of Cu and Fe in nature is susceptible to isotope fractionation processes during the biogeochemical cycle. Since Cu and Fe are redox sensitive metals, differences in their oxidation states can lead to variations in the stable isotope composition of the aquatic species or compounds that they form. Stable isotopes of Cu and Fe have recently been used to trace metal redox cycles, nutrient pathways, metal contaminant sources and to develop isotopic biosignatures. The objective of this project was to study the geochemical processes governing the isotopic fractionation of Cu and Fe in mine impacted sites, including processes related to mineral processing. One of the key questions was to explain the role of bacteria in the variations of the isotopic composition of Cu and Fe. First, bioleaching and electroleaching of a chalcopyrite concentrate were performed. During the chalcopyrite leaching in both experiments the first release of Cu to the leachate is enriched in the heavier Cu isotope as a product of oxidative dissolution. At the later stages of leaching, the δ65Cu values for the leachate are similar to the initial material, confirming an equilibrium fractionation in a closed system. In the case of Fe isotope fractionation the dissolution of pyrite at redox potentials higher than 600mV leads to an enrichment of the heavier Fe isotope in the leachate in the bioleaching experiment, mainly regulated by the formation of secondary minerals such as jarosite. Soil bacteria were studied in three different experimental scales using pot, lysimeters and field experiments, amended with autochthonous plant growth promoting bacteria. Roots and plants from pots showed no variation in their Fe and Cu isotope composition compared to non-amended samples. However, plants growing in the amended substrates regardless of their experimental scale, showed variations in the Fe and Cu isotope composition of their roots with an increase in the heavier Cu isotope. Siderophores released either by bacteria or the plant can complexate available Cu and Fe in the soil, causing a change in the isotope fractionation of those metals. The second question is related to the biogeochemical cycle of Cu and Fe. In mine tailings the sulphide oxidation resulted in an enrichment of the lighter Cu isotope in secondary phases in the oxidized zone of the tailings compared to the original isotope composition in the unoxidised mineral. Precipitation of covellite at the oxidation front of the tailings profile resulted in a significant enrichment of the lighter Cu isotope in the bulk soil with a δ65Cu value as low as -4.35 ±0.02 ‰. Fe isotope fractionation in the Kristineberg test cell varied due to processes such as Fe(II)-Fe(III) equilibrium and precipitation of Fe-(oxy)hydroxides at the oxidation front, where δ56Fe values were higher than in the initial material. As a way to link the obtained results from this thesis, a self-restored mine site was studied. A variation towards higher δ65Cu values was seen from rocks, to water and biofilms. Cu absorption mainly by extrapolymeric substances and secondary mineral precipitation regulates the isotopic composition of the biofilm. Oxidative weathering of sulphide minerals and further precipitation of Fe-(oxy)hydroxides are considered to be the main causes for Fe isotope fractionation in this area. Summing up, this thesis provides several field studies to corroborate the data observed in the lab regarding processes that are important for the biogeochemical cycling of metals and could be further applied to the extraction of metals or for remediation purposes.